The present invention generally relates to reactive plasma sputter deposition techniques for forming and depositing films on substrates and, more particularly, is concerned with a system and method for sputter deposition of a material on a substrate using dual cathode or dual anode systems.
Sputter deposition is a process wherein a target, usually a metal, is placed in position near a plasma (a cloud of ions and electrons in equal numbers), in a chamber in which most of the air has been withdrawn. Well-known conventional means are used to create the plasma. A negative voltage is produced on the target, or cathode, relative to a separate electrode called the anode by connecting the negative lead of a dc power supply to the target. The negative voltage on the target attracts the ions from the plasma, which are accelerated toward the target. Upon arrival the collision of the ions with the target physically knocks out target atoms. These target atoms travel from the target to a substrate placed nearby, which becomes coated with them. The expelled target atoms also coat every other surface in the system, as for the most part they are neutral and there is no practical way to direct their path. When ions are withdrawn from the plasma, there immediately exists an excess of electrons in the plasma. These excess electrons are attracted to the positive lead of the dc power supply used to create the target voltage, which positive lead is connected to a separate electrode called the anode or alternatively to the chamber walls, either of which, in collecting the electrons, provide for plasma current flow and therefore may be considered as plasma current providing elements.
As described, this is a very common process for deposition of thin layers of metals. It is widely used in the processing of semiconductors, and in creating the reflecting layer on compact discs and CD-ROMS, active layers on hard discs for computer storage, and layers of metals for many other functional and decorative applications.
The process described above is called dc sputtering, and requires that the target (or cathode) be conducting, because the ions arriving at the target must be able to accept one or more electrons from the target to become neutral gas atoms again in order to prevent charging of the target surface, which would create a retarding potential which would stop the process very quickly. Insulators do not have free electrons available for this purpose, so that an insulating target material cannot be used. On the other hand, one can deposit layers of insulating material from a metallic target, by forming the insulator chemically through reaction with a reactive background gas. This is called reactive sputtering. For example, Al2O3 and SiO2 can be created from aluminum and silicon targets, respectively, if oxygen gas is present in appropriate quantities in the background gas filling the chamber.
There is increasing commercial interest in processes involving deposition of such insulating films. This interest comes about at least in part because of the application of such processes to the deposition of wear resistant coatings; insulating films for microcircuits (including devices such as thin film heads) or electronic devices such as capacitors; sophisticated architectural glass coatings; coatings on polyester film for architectural glass laminates or oxygen barriers for food packaging; heat reflecting coatings for high efficiency lamps or induction furnace heat shields; deposition of barrier and functional layers for flat panel displays, including the ITO glass used in LCD displays; and myriad other similar functional applications. Added to this are the many reactive PVD processes used to create decorative effects on a wide variety of plastic, natural and artificial fiber, and metal substrates.
A problem occurs in these and other cases, however, when the reaction product is an electrical insulator. Since, as described above, the insulating film coats every surface in the chamber (which it will eventually do) then it will surely eventually coat the anode. As this happens the conduction path for the electrons is coated over, and the process cannot be sustained. This has been termed the xe2x80x9cdisappearing anodexe2x80x9d problem. In the past the reactive process was run until this effect began to create serious problems, whereupon the system was opened to mechanically scrub off the offending insulating layer from the anode to create a new metallic surface. Thus, continuous operation without this maintenance was not possible.
Another drawback related to the coating of the anode with an insulator is that this insulator will generally charge up as the electrons attempt to collect there. This charge can cause an electric field in the insulating film on the anode which may exceed the dielectric strength of the film material. When this occurs an arc may be formed and the energy in this arc may cause portions of the film to be ejected from the anode, creating particles which can become included in the film growing on the substrate, causing defects which may be unacceptable in the final product.
Este, et al, in an article entitled xe2x80x9cA Quasi-direct-current Sputtering Technique for the Deposition of Dielectrics at Enhanced Ratesxe2x80x9d, published in J. Vac. Sci. Technol. A, vol. 6, No. 3 (May/June 1988), proposed an approach to the sputtering process which uses two targets alternately for deposition of dielectric or insulating films. The power supply, which in this case has an alternating potential output, is connected to the two targets so that they are driven alternatively positive and negative with respect to one another. This causes each to act as an anode for the other. If the reversal takes place often enough, only a very thin layer of insulator will be formed on the target acting as an anode, and this very thin layer can be sputtered away when it is that target""s turn to be negative. This is possible because the insulator does not stop the sputtering process at once, but due to charging effects its presence will slow and eventually stop the process. If the layer is very thin, it can be sputtered away before the process stops. The usual time for reversal is a few tens of microseconds, in order that there be too little time for a thick layer to form. See also the paper by Schiller et al entitled xe2x80x9cPulsed Magnetron Sputter Technologyxe2x80x9d, published in the Proceedings of the 1993 International Conference on Metallurgical Coatings and Thin Films, Surf. Coat. Tech. Vol. 61, (1993) page 331, which covers a dual magnetron target approach similar to that of Este et al in that each of the targets acts within one cycle of the output of the power supply once as the cathode and once as the anode. This method is generally called xe2x80x9cdual cathode sputteringxe2x80x9d.
For the most part this has proved to be a successful approach to the problem of the xe2x80x9cdisappearing anodexe2x80x9d. It does have the disadvantage, however, of requiring two targets, which adds to the expense of the system and also complicates the maintenance. Also, it is difficult to retrofit this dual target process into existing sputtering systems because there often is no room for the second target. In addition, large anode potentials in such a system due to the magnetic field over the target cause a loss in deposition rate and higher substrate heating.
Another approach to the problem is described in U.S. Pat. No. 5,897,753, issued Apr. 27, 1999, entitled xe2x80x9cContinuous Deposition of Insulating Material using Multiple Anodes Alternated between Positive and Negative Voltagesxe2x80x9d, hereby incorporated by reference. In this approach two or more anodes are provided and driven alternatively positive and negative, permitting each to be sputtered and therefore cleaned once each cycle. This method, when two anodes are provided, may be called xe2x80x9cdual anode sputteringxe2x80x9d, and addresses some of the problems of dual cathode sputtering.
Now it can be important, in order to obtain dense high quality films, to cause the growing film to be bombarded by ions. This ion bombardment can increase the density and quality of the film by increasing the surface energy of the film, causing the arriving atoms to more easily find the lowest energy point in the growing lattice of atoms. In conventional sputtering with a single target and single anode, this ion bombardment can be provided by causing the substrate to be held at a negative potential relative to the anode, and thus to the plasma (in the conventional single target configuration, the plasma is usually slightly more positive than the anode). This negative substrate potential may attract ions from the plasma, providing the desired bombardment. If the growing film is electrically conductive, this substrate bias may be easily provided by connecting a dc power supply between the substrate and the anode. If the film is insulating, a high frequency supply may be connected in the same way to produce a negative surface potential through preferential electron attraction at the peaks of each cycle since electrons are lighter and more mobile than ions. In either case, the connection to the anode is effective because this element is usually close in potential to the plasma.
Both the dual cathode and dual anode approach present a problem for the user who wishes to enhance ion bombardment of the substrate, because in both approaches the powering system is floating with respect to the chamber, and there is no single electrode which remains close in electrical potential to the plasma.
Consequently a need exists for a method of providing for the substrate a consistent controllable negative electrical potential relative to the plasma, and thus provide a means of controllable ion bombardment of the substrate.
It is an object of the present invention, therefore, to provide a means of biasing the substrate in a dual anode or dual cathode sputtering system.
It is a further object of this invention to provide biasing of the substrate in a dual anode or dual cathode sputtering system without the need for a separate substrate biasing supply.
It is yet another object of the present invention to provide biasing of the substrate in a dual anode or dual cathode sputtering system in a simple, reliable manner regardless of the electrical conductivity of the growing film on the substrate.
Accordingly, the present invention is directed to a system for sputter deposition of an electrically insulating or conducting material on a substrate while simultaneously bombarding the depositing film with ions. The present invention discloses a novel connection to the substrate or substrate holder to permit this, using the same power supply as is used to power the cathode or cathodes, with or without an auxiliary biasing supply.
In one embodiment, perhaps employing the dual cathode arrangement, a transformer is provided with a center tap in its secondary which may be used to produce a pulsating negative voltage for biasing the substrate.
In another embodiment, perhaps employing the dual anode arrangement, a novel connection to the cathode through a dropping resistor may be used to provide the required negative potential relative to the plasma.
In any of these embodiments, it may be desirable for the substrate to be held negative with respect to the plasma so that it may be bombarded with ions, at least during part of the cycle of coating, and preferably for a large percentage of the cycle time. This may be done intrinsically, or by the addition of an auxiliary power supply, and this auxiliary supply may produce direct current power or alternating power.